Bleaching and delignifying cellulose using caroate/caro&#39;s acid and production of same

ABSTRACT

The invention relates to a method of bleaching and/or delignifying cellulose, in which  
     (a) a sodium hydroxide solution and a peroxydisulphate solution are first of all produced by electrolysis of a sulphate,  
     (b) the peroxydisulphate solution is converted by hydrolysis into a Caro&#39;s acid/caroate solution,  
     (c) said Caro&#39;s acid/caroate solution is left to cool and  
     (d) used immediately as a bleaching solution for bleaching and/or delignifying cellulose, the Caro&#39;s acid/caroate solution becoming neutralised upon introduction into the bleaching solution.

[0001] This invention relates to a method of bleaching and/ordelignifying cellulose using Caro's acid/caroate solution.

[0002] The traditional method of bleaching wood pulp for paper is to uselarge quantities of chlorine. However, since chlorinated lignin ranksamong the environmentally toxic polychlorinated aromatics, increasingefforts have been made to replace the bleaching agent chlorine withother bleaching agents. The use of chlorine dioxide/chlorate asbleaching agent marked the start of a successful new era. Continuingefforts, however, are focused on replacing chlorine dioxide/chloratewith yet other bleaching agents, and thus to obtain TCF (Total ChlorineFree) pulp.

[0003] Bleaching agents which have been used to manufacturechlorine-free paper include ozone, oxygen and hydrogen peroxide.However, all these bleaching agents have disadvantages, such asinsufficient delignification, insufficient brightness of the bleachedcellulose and/or cost-intensive and/or dangerous production methods.

[0004] The use of caroate/Caro's acid (peroxymonosulphuric acid, H₂SO₅)as a bleaching agent for the bleaching and delignification of cellulosehas also been suggested, especially for the chlorine-free bleaching offine paper required to have a brightness level of 89 on the ISO scale. Adisadvantage of using caroate/Caro's acid as bleaching agent, however,is the tedious process required to obtain the pure salt as a crystallineproduct. Pure Caro's acid, moreover, is an extremely corrosivesubstance, which makes storage and transport difficult.

[0005] The U.S. Pat. No. 5,366,593 describes a method of bleachinglignocellulose-containing material, in which, as reactants, the oxygendonor monoperoxy-sulphate and a ketone are used to form a water-solubledioxirane on-site, which then acts as a bleach. The direct use ofcaroate/Caro's acid is not considered.

[0006] The U.S. Pat. No. 4,144,144 describes the production of sodiumperoxydisulphate in an electrolytic cell, where the current efficiencycan be increased by a suitable choice of anolyte.

[0007] The EP-A-O 415 149 A2 describes a method of bleaching anddelignifying lignocellulose materials using peroxymonosulphuric acidand/or salts thereof in combination with oxygen and/or a peroxide. Theperoxymonosulphuric acid is used in the form of its salt, or is obtainedby reacting hydrogen peroxide with concentrated sulphuric acid. Asimilar process, in which peroxymonosulphuric acid is produced byreacting hydrogen peroxide and sulphuric acid, is also described by B.Amini et al. in Tappi Journal, Vol. 78, No. 10 (1995), 121-133.

[0008] The U.S. Pat. No. 4,049,786 describes a method of producingperoxymonosulphate by rapid hydrolysis of a peroxydisulphate withconcentrated sulphuric acid, where active oxygen loss during hydrolysiscan be suppressed by a suitable choice of reaction temperature.

[0009] WO-A-94/05851 describes a method of bleachinglignocellulose-containing pulp using peroxydisulphate.

[0010] The object of this invention was thus to provide a method withwhich the beneficial bleaching and delignification effects of Caro'sacid/caroate can be exploited and, at the same time, the difficultiesconnected with the production of pure Caro's acid/caroate overcome andthe danger of corrosion in storage and transport devices avoided.

[0011] This object is established according to the invention by a methodof bleaching and/or delignifying cellulose, especially paper, in which

[0012] (a) a sodium hydroxide solution and a peroxydisulphate solutionare first of all produced by electrolysis of a sulphate,

[0013] (b) the peroxydisulphate solution is converted by hydrolysis intoCaro's acid/caroate solution,

[0014] (c) said Caro's acid/caroate solution is left to cool and

[0015] (d) used immediately as a bleaching solution for bleaching and/ordelignifying cellulose, especially paper, the Caro's acid/caroatesolution becoming neutralised upon introduction into the bleachingsolution.

[0016] The method of the invention allows the use of Caro's acid/caroatein the form of a solution, which means that the disadvantages connectedwith the pure crystalline product can be avoided. These disadvantagesare, in particular, the tedious process required to obtain the pure saltin crystalline form, and the corrosive nature of Caro's acid.

[0017] It is an essential feature of the invention that Caro'sacid/caroate solution can be produced either in, or in the immediatevicinity of a pulp or paper mill, so that long transport distances—whichnecessitate separating off the water from the solution and obtaining theproduct in pure form—can be avoided. Protracted storage or transportover long distances require that the water first be separated off toform pure Caro's acid/caroate because, on the one hand, the pure productis in itself more stable than a solution thereof, which means it can bestored for longer, and on the other hand, the proportion of water in thesolution makes transport over long distances considerably more difficultand expensive.

[0018] Production of Caro's acid/caroate either in, or in the immediatevicinity of a pulp or paper mill is realized by first of all producing asodium hydroxide solution and a peroxydisulphate solution, for examplesodium peroxydisulphate, potassium peroxydisulphate and/or calciumperoxydisulphate, by electrolysis of a sulphate, for example sodiumsulphate, potassium sulphate and/or calcium sulphate. Theperoxydisulphate solution can then be converted by hydrolysis intoCaro's acid/caroate solution. Whereas this production process can becarried out without any danger in the proximity of wood, cellulose orpaper, the conventional production of caroate from oleum and 70%hydrogen peroxide is extremely risky, since any contact between thestarting materials and wood, cellulose or paper will cause a fire. Thisis the reason why it was hitherto not possible to produce Caro'sacid/caroate in the vicinity of a pulp or paper mill.

[0019] Another advantage of simultaneously producing a peroxydisulphatesolution and a sodium hydroxide solution by means of electrolysis isthat pulp and paper mills often have inexpensive electricity fromhydroelectric power stations, and that waste sulphate from sulphatecellulose can be put to economic use. It is preferable if the proportionof waste sulphate from sulphate cellulose accounts for more than 5 wt. %of the total amount of sulphate used in step (a), especially preferableif it accounts for more than 50 wt. % and most preferable of all if itaccounts for more than 90 wt. % thereof.

[0020] According to the invention, the peroxydisulphate solutionproduced by electrolysis is then converted by way of hydrolysis intoCaro's acid/caroate solution. On cooling, this Caro's acid/caroatesolution was found to be relatively stable. Through addition of H₂O₂during hydrolysis, the decomposition of Caro's acid/caroate can beminimised further. According to the invention, the Caro's acid/caroatesolution obtained is used immediately as a bleaching solution forbleaching and/or delignifying cellulose, especially paper. The Caro'sacid/caroate solution can be introduced—for example by way of a directpipeline or in form of a bulk transport—into the bleaching solution. Itis thus unnecessary, according to the invention, to produce Caro'sacid/caroate in crystalline form.

[0021] According to the invention, the Caro's acid/caroate solution isneutralised upon introduction into the bleaching solution. It isbeneficial to neutralise the Caro's acid/caroate solution at this pointin time because a pH <5 promotes the stability of Caro's acid/caroatesolution. Another reason is that the heat of neutralisation isdistributed in the bleaching solution, thus reducing decomposition ofthe caroate solution/ Caro's acid still further.

[0022] As already mentioned, it is essential to the invention that themethod of the invention can be carried out either in, or in theimmediate vicinity of a pulp or paper mill. An added advantage of suchan arrangement is that large paper mills usually already have achlorine-alkali electrolysis facility, which can be modified to meet therequirements of a peroxydisulphate electrolysis. Furthermore, during thekraft process, for example, some 5 to 20 kg of sodium sulphate are addedper tonne of paper. Before being supplied to the pulping process, thissodium sulphate can be used in the peroxydisulphate electrolysis. Thesulphate, which is recovered after the bleaching process, is usedsubsequently in the pulping process. In this way, the sodium sulphaterequired anyway for the pulping process is first used for the bleachingprocess of the invention, after which it is recovered. The recoveredsodium sulphate is sufficiently pure to be used subsequently in thepulping process.

[0023] Another advantage of the method of the invention is that it doesnot require the use of additional chemicals, so that there are noattendant problems with respect to disposal or environmentalcompatibility.

[0024] A preferred feature of the invention is the use of the sodiumhydroxide solution produced by electrolysis to neutralise the Caro'sacid/caroate solution in the bleaching solution.

[0025] It is furthermore preferable to use a membrane cell or adiaphragm cell—of the type described, for example, in Ullmann, Vol. 19,page 216—for the electrolysis.

[0026] It is preferable if the hydrolysis of the peroxydisulphatesolution, during which a Caro's acid/caroate solution is formed, isconducted at 50 to 90° C., and especially preferable if it is conductedat 60 to 80° C. for 30 to 60 minutes, preferably 30 to 40 minutes. Forthe hydrolysis, it is preferable to use 1 to 4 N sulphuric acid. Afterhydrolysis, the Caro's acid/caroate solution, which preferably has a pHof <5, is allowed to cool to 0° C. to 50° C., preferably to 10° C. to30° C.

[0027] The sulphate formed during the bleaching and/or delignificationof cellulose, especially paper, can be recovered and supplied, forexample, to a sulphate pulping process. The invention provides for theuse of any suitable sulphate for electrolysis, preference being given tosodium sulphate, potassium sulphate and/or calcium sulphate.

[0028] With the method of the invention, cellulose, especially paper,can be obtained with a high level of brightness, measuring 89 on the ISOwhiteness scale. A preferred bleaching sequence is, for example:

O—Q—P——X—P*,

[0029] where O is oxygen, Q a complexing agent such as EDTA, P is H₂O₂and X—P* is Caro's acid/caroate solution. However, other bleachingsequences may also be used.

[0030] The invention is explained in more detail by means of thefollowing examples.

EXAMPLE 1 Production of Caro's Acid

[0031] Caro's acid was produced by hydrolysis from a solution of sodiumperoxydisulphate. To this end, 2 mol/l (4N) SPS (sodiumperoxydisulphate), ie, 476 g/l, and 0.25 M sulphuric acid (25.5 g/l)were heated to 70° C., with stirring, and kept at this temperature withcontinued stirring. The composition of the solution before and afterhydrolysis is recorded in Table 1: TABLE 1 Time SPS H₂SO₅ H₂SO₄ Beforeheating 476 g/l 0 g/l 25.5 g/l After 3 hours 59.9 g/l 107 g/l 210.7 g/l

[0032] The yield of H₂SO₅ after 3 hours was 46.9%. After the solutionhad been cooled to room temperature, the decrease in caroate contentover 20 hours was found to be only 2.8% relative (104 g H₂SO₅/l).

[0033] In another experiment, 2 mol/l (4N) NPS, ie, 476 g/l, and 2Nsulphuric acid (98 g/l) were heated to 70° C., with stirring, and keptat this temperature with continued stirring. The composition of thesolution before and after hydrolysis is recorded in Table 2: TABLE 2Time SPS H₂SO₅ H₂SO₄ Before heating 476 g/l 0 g/l 98 g/l After 1 hour9.5 g/l 162.3 g/l 246 g/l

[0034] The yield of H₂SO₅ after 1 hour was 71.2%. After the solution hadbeen cooled to room temperature and partially neutralised to 0.34 NH₂SO₄, the caroate content only decreased by 1.3% relative.

EXAMPLE 2

[0035] Comparative bleaching and delignifying with PMPS (potassiummonoperoxy-sulphate), hydrogen peroxide, and the caroate of example 1

[0036] Two Finnish pulp samples were analysed: Sample 5656 contains68.5% water, a biocidal additive, and has the kappa number 20.3(lignin). Sample 5657 contains 60.9% water, a biocidal additive, and hasthe kappa number 12.8 (lignin). Sample 5657 has already been pre-treatedwith oxygen. It is therefore brighter and contains less lignin.

Experiment a)

[0037] Pulp sample 5656 (not pre-bleached) was bleached with PMPS. Thepulp density was 3%. For the experiment, 13% abs. dry KHSO₅ (29.6 gCuroxe®/100 g dry pulp) was used, which corresponds to 10% abs. dryH₂SO₅. The bleaching effect was visible immediately after addition.

[0038] After 30 minutes' treatment at 60° C. and pH 2.5, the sample wasfiltered under suction and washed with a large volume of water. Afterthe sample had been dried carefully overnight at 30° C., brightness andweight were determined.

Experiment b)

[0039] Pulp sample 5656 (not pre-bleached) was bleached with hydrogenperoxide.

[0040] The pulp density was 3%. For the experiment, 2.9% abs. dry H₂O₂was used. This amount has the same active oxygen content (corresponds to1.4% abs. dry AO) as the 10% abs. dry H₂SO₅ (experiment a).

[0041] After 30 minutes' treatment at 60° C. and pH 6, the sample wasfiltered under suction and washed with a large volume of water. Afterthe sample had been dried carefully overnight at 30° C., brightness andweight were determined.

Experiment c)

[0042] Pulp sample 5656 (not pre-bleached) was bleached with a solutionof 116.4 g H₂SO₅ obtained according to Example 1.

[0043] The pulp density was 3%. For the experiment, 10% abs. dry H₂SO₅was used. The active oxygen content in this amount corresponds to 1.4%abs. dry AO.

[0044] After 30 minutes' treatment at 60° C. and pH 1.5, the sample wasfiltered under suction and washed with a large volume of water. Afterthe sample had been dried carefully overnight at 30° C., brightness andweight were determined.

Experiment d)

[0045] Pulp sample 5657 (not pre-bleached) was bleached with PMPS.

[0046] The pulp density was 3%. For the experiment, 13.3% abs. dry KHSO₅(29.6 g Curox®/100 g dry pulp) was used, which corresponds to 10% abs.dry H₂SO₅.

[0047] After 30 minutes' treatment at 60° C. and pH 2.5, the sample wasfiltered under suction and washed with a large volume of water. Afterthe sample had been dried carefully overnight at 30° C., brightness andweight were determined.

Experiment e)

[0048] Pulp sample 5657 (pre-bleached) was bleached with hydrogenperoxide.

[0049] The pulp density was 3%. For the experiment, 2.9% abs. dry H₂O₂were used. This amount has the same active oxygen content (correspondsto 1.4% abs. dry AO) as the 10% abs. dry H₂SO₅.

[0050] After 30 minutes' treatment at 60° C. and pH 6, the sample wasfiltered under suction and washed with a large volume of water. Afterthe sample had been dried carefully overnight at 30° C., brightness andweight were determined.

Experiment f)

[0051] Sample 5657 (not pre-bleached) was bleached with a solution of116.4 g H₂SO₅ obtained according to example 1.

[0052] The pulp density was 3%. For the experiment, 10% abs. dry H₂SO₅was used. The active oxygen content in this amount corresponds to 1.4%abs. dry AO.

[0053] After 30 minutes' treatment at 60° C. and pH 1.5, the sample wasfiltered under suction and washed with a large volume of water. Afterthe sample had been dried carefully overnight at 30° C., brightness andweight were determined.

Results of Bleaching Experiments a) to f)

[0054] Pulp Degree of Experiment Sample Abs. dry AO density brighteningKappa a) 5656 29.6% PMPS 1.4% 3% +9.2% 18.4 = −12.2% b) 5656  2.9% H₂O₂1.4% 3% +5.2% 0 c) 5656  10% H₂SO₅ 1.4% 3% +7.6% 18.9 = −9.6%  d) 565729.6% PMPS 1.4% 3% +14.3%  12.9 = −12.4% e) 5657  2.9% H₂O₂ 1.4% 3%+13.9%  0 f) 5657  10% H₂SO₅ 1.4% 3% +13.9%  11.3 = −12.4%

1. A method of bleaching and/or delignifying cellulose, in which (a) asodium hydroxide solution and a peroxydisulphate solution are first ofall produced by electrolysis of a sulphate (b) the peroxydisulphatesolution is converted by hydrolysis into Caro's acid/caroate solution(c) said Caro's acid/caroate solution is left to cool and (d) usedimmediately as a bleaching solution for bleaching and/or delignifyingcellulose, the Caro's acid/caroate solution becoming neutralised uponintroduction into the bleaching solution.
 2. The method of claim 1characterized in that the sodium hydroxide produced in step (a) is usedfor neutralisation in step (d).
 3. The method of claims 1 or 2,characterized in that a membrane cell or a diaphragm cell is used forelectrolysis in step (a).
 4. A method according to one of the precedingclaims, characterized in that the hydrolysis of the peroxydisulphatesolution in step (b) is carried out at 50° C. to 90° C.
 5. A methodaccording to one of the preceding claims, characterized in that thehydrolysis of the peroxydisulphate solution in step (b) is carried outin 1 to 4 N sulphuric acid.
 6. A method according to one of thepreceding claims, characterized in that the Caro's acid/caroate solutionis allowed to cool to 0° C. to 50° C.
 7. A method according to one ofthe preceding claims, characterized in that sulphate formed during thebleaching and/or delignification of cellulose is recovered.
 8. A methodaccording to one of the preceding claims, characterized in that asulphate selected from the group comprising sodium sulphate, potassiumsulphate and/or calcium sulphate is used.